Early transition metal stabilized high capacity cobalt free cathodes for lithium-ion batteries

ABSTRACT

A cathode for a lithium battery includes LiNi0.5-x/2Mn0.5-x/2MxO2 where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is between 0.005-0.02. The LiNi0.5-x/2Mn0.5-x/2MxO2 can be coated with Mn2P2O7. The Mn2P2O7 can be 1-3 wt. %, based on the total weight of the LiNi0.5-x/2Mn0.5-x/2 MxO2 and Mn2P2O7. A cathode composition, a lithium battery, and a method of making a lithium battery are also disclosed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to cathodes for lithium ion batteries, and more particularly to cobalt-free cathodes for lithium ion batteries.

BACKGROUND OF THE INVENTION

One major technical barrier limiting the energy density of lithium-ion batteries is the lack of robust, high-capacity cathodes. When charged beyond ˜4.3 V vs. Li/Li⁺, traditional layered LiMO₂ cathodes (M=Mn, Co, Ni) undergo irreversible structural transformations with concomitant oxygen loss, leading to capacity and voltage fade upon cycling. Understanding and addressing these structural instabilities is vitally important to design next-generation cathodes which can better utilize their Li supply without sacrificing cycle life.

Various cation substitutions have been explored to improve the performance of LiMO₂ cathodes. Two notable examples include Ni-rich NMC (LiNi_(x)Mn_(y)Co_(1-x-y)O₂, x≥0.5) and NCA (for example LiNi_(0.8)Co_(0.15)Al_(0.05)O₂) which have reversible capacities as high as 200 mAh/g. Despite their significant improvements over LiCoO₂ (˜140 mAh/g), NMC and NCA still only utilize ˜70% of their Li reserve. An alternative strategy to stabilize the cathode structure at high states of charge is to design composite cathode structures with LiMO₂ as the primary Li storage site and with Li₂M′O₃ as a stabilizing unit. The most prominent among these materials are the layered-layered composites normally referred to as Li—Mn-rich NMC (LMR-NMC, xLi₂MnO₃.(1-x)LiNi_(y)Mn_(z)Co_(1-y-z)O₂). After an electrochemical activation step in which Li and O are removed from the Li₂MnO₃, extremely high reversible capacities ˜250 mAh/g can be achieved. However, the viability of LMR-NMC cathodes is limited by Mn dissolution in the electrolyte and voltage fade which occur during extended cycling. First-principles calculations have suggested that the voltage fade of these materials is caused by irreversible migration of Mn to the Li-layer during the first charging cycle.

Li₂MoO₃ has recently been proposed as a candidate to replace Li₂MnO₃ in layered-layered composite cathodes, although the synthesis of these Mo-containing composites has not been reported to date. Li₂MoO₃ has R-3m symmetry and thus is expected to coherently blend with traditional layered LiMO₂ materials. Furthermore, Mo can access multiple oxidation states such as Mo⁴⁺—Mo⁶⁺, allowing for reversible Li storage in both the Li₂MoO₃ and LiMO₂ moieties. Finally, Li₂MoO₃ may have improved oxidative stability compared to Li₂MnO₃ due to reversible anionic charge compensation. Oxygen redox activity in Li-excess layered materials originates from unique Li—O—Li configurations in which electrons can be extracted from unhybridized O 2p states.

Cobalt has been an important component of cathodes for lithium batteries. The limited supply of this material has led to it being one of the most expensive supply materials of the battery manufacturing process. Limiting the need for cobalt in the battery system has therefore become a requirement for improving the affordability and accessibility of electric vehicles and devices.

SUMMARY OF THE INVENTION

A cathode for a lithium battery includes LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is between 0.005-0.02. The LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ can be coated with Mn₂P₂O₇. The Mn₂P₂O₇ can be 1-3 wt. %, based on the total weight of the LiNi_(0.5-x/2)Mn_(0.5-x/2) M_(x)O₂ and Mn₂P₂O₇. The cathode can be cobalt free. The M can be substituted at cation sites. The cathode can further comprise a conductive carbon and a binder. The usable capacity of the cathode can attain 180 mAh/g when cycled to 4.5 V vs. Li/Li⁺.

The LiNi_(0.5-x/2) Mn_(0.5-x/2) M_(x)O₂ can be provided as particles having a diameter of from 50 to 500 nm. The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ can be coated with Mn₂P₂O₇, and the coating can be between 1 and 5 nm thick.

The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ can be a solid solution wherein the lithium concentration is in excess between 15 to 30 at % to form a lithium excess composition (i.e., Li_(1+y)Ni_((1-y)(0.5-x/2))Mn_((1-y)(0.5-x/2))M_((1-y)x)O₂ where y is between 0.15-0.30 and x is between 0.005-0.02).

A cathode composition for a lithium battery includes LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of M=Mo, Ti, Cr, Zr and V, and x is 0.005-0.02. The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ can be provided as particles, and the particles can be coated on all sides with Mn₂P₂O₇.

A lithium battery can include a cathode comprising LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is 0.005 to 0.02; an anode; a separator; and an electrolyte. The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, can be provided as particles, and the particles can be coated on all sides with Mn₂P₂O₇. The usable capacity of the cathode can attain 180 mAh/g when cycled between 2.0-4.5 V vs. Li/Li⁺.

A method of making a lithium battery can include the steps of providing LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is 0.005 to 0.02; mixing the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ with conductive carbon, a binder and a solvent to form a slurry, and casting the slurry onto a current collector and drying the slurry to form a cathode; and combining the cathode with an anode, a separator and an electrolyte to form a battery. The binder can be PVDF. The solvent can be N-Methyl-2-Pyrrolidone (NMP). The electrolyte can be 1.2 M LiPF₆ in EC-EMC in 3:7 weight ratio. The anode composition can be a mixture of graphite and a binder.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferred it being understood that the invention is not limited to the arrangements and instrumentalities shown, wherein:

FIG. 1 is a schematic drawing of the crystal structure of LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂.

FIG. 2 is a graph of intensity (a.u) v. 2θ_(CuKα)/deg. showing the Mo doped LiNi_(0.5)Mn_(0.5)O₂ X-ray diffraction pattern.

FIG. 3 is a SEM image of a baseline LiNi_(0.5)Mn_(0.5)O₂ without Mo doping.

FIG. 4 is a SEM image of Mo doped LiNi_(0.5)Mn_(0.5)O₂.

FIG. 5 is a graph of specific capacity (mAh/g) v. cycle number (n) for LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂, x=0 and x=0.01 cycled between 2.0-4.5 V vs. Li/Li⁺ at a specific current of 20 mA/g.

FIG. 6 is a graph of specific capacity (mAh/g) v. cycle number (n) for LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂ x=0 and x=0.01 cycled between 2.0-4.5 V vs. Li/Li⁺ at specific current density of 10-200 mA/g.

FIG. 7 is a graph of voltage vs. Li/Li⁺/V vs. specific capacity (mAh/g) showing electrochemical characterization for baseline LiNi_(0.5)Mn_(0.5)O₂.

FIG. 8 is a graph of voltage vs. Li/Li⁺/V vs. specific capacity (mAh/g) for LiNi_(0.495)Mn_(0.495)Mo_(0.01)O₂ collected a specific current density of 20 mA/g.

FIG. 9 is a graph of specific capacity (mAh/g) v. cycle number (n) for LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ cycled between 2.0-4.2 V vs. Li/Li⁺ at a specific current density of 20 mA/g.

FIG. 10 is graph of specific current (mA/g_(active material)) V. voltage (V) for LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂ and x=0 and x=0.01 collected at a scan rate of 0.1 mV/s.

FIG. 11 is a graph of normalized counts/A.U v. Raman shift/cm⁻¹ before and after 100 cycles for a baseline LiNi_(0.5)Mn_(0.5)O₂.

FIG. 12 is a graph of normalized counts/A.U v. Raman shift/cm⁻¹ before and after 100 cycles for LiNi_(0.495)Mn_(0.495)Mo_(0.01)O₂.

FIG. 13 is a graph of intensity (a.u) v. 2θ_(CuKα)/deg. showing the X-ray diffraction pattern for Mo doped LiNi_(0.5)Mn_(0.5)O₂ cathode coated with Mn₂P₂O₇.

FIG. 14 is a graph of specific capacity (mAh/g) v. cycle number (n) for LiNi_(0.5)Mn_(0.5)O₂ (LNMO), LiNi_(0.495)Mn_(0.495)Mo_(0.01)O₂ (LNMMO), and Mn₂P₂O₇-LNMMO, showing the electrochemical characterization for Mn₂P₂O₇ coated on a Mo doped LiNi_(0.5)Mn_(0.5)O₂ cathode, cycled between 2.0-4.5 V vs. Li/Li⁺ at specific current density of 20-50 mA/g.

FIG. 15 is a graph of specific capacity (mAh/g) v. cycle number (n) for LNMO, LNMMO, and Mn₂P₂O₇ coated on a Mo doped LiNi_(0.5)Mn_(0.5)O₂ cathode, cycled between 2.0-4.2 V vs. Li/Li⁺ at a specific current density 20 mA/g.

FIG. 16 is a graph of specific capacity (mAh/g) v. cycle number (n) showing electrochemical characterization for Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂, cycled between 2.0-4.5 V vs. Li/Li⁺ at a specific current density 20 mA/g.

FIG. 17 is a graph of specific capacity (mAh/g) v. cycle number for LNMO, LNMMO, and Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂, cycled between 2.0-4.5 V vs. Li/Li⁺ at specific current density of 10-200 mA/g.

FIG. 18 is a graph of potential (V vs. Li/Li⁺) v. specific capacity (mAh/g) for Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂ collected at a specific current density of 20 mA/g.

FIG. 19 is a graph of specific capacity (mAh/g) v. cycle number (n) for Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂ cycled between 2.0-4.2 V, 2.0-4.5 V, 2.0-4.6 V, and 2.0-4.8 V at a specific current density of 20 mA/g.

DETAILED DESCRIPTION OF THE INVENTION

A cathode for a lithium battery includes LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is between 0.005-0.02. The LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ can be coated with Mn₂P₂O₇. The Mn₂P₂O₇ can be 1-3 wt. %, based on the total weight of the LiNi_(0.5-x/2)Mn_(0.5-x/2) M_(x)O₂ and Mn₂P₂O₇.

The LiNi_(0.5-x/2) Mn_(0.5-x/2) M_(x)O₂ can be provided as particles having a diameter from 50 to 500 nm. The diameter of the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ particles can be 50, 75, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, or 500 nm, or within a range of any high value and low value selected from these values.

The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ particles can be coated with Mn₂P₂O₇. The Mn₂P₂O₇ coating on the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ can cover all sides of the particles. The Mn₂P₂O₇ coating can be 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75, 4, 4.25, 4.5, 4.75 or 5 nm thick, or can be within a range of any high value and low value selected from these values.

The cathode of the invention can be cobalt free. The cathode can further contain a conductive carbon and a binder. Any suitable conductive carbon can be used. The specific surface area of many carbon blacks is typically in the range of 50-250 m²/g. Two common carbon additives for LIB cathodes include Super P Li and C65 (Imerys Graphite and Carbon, Bironicom Switzerland). The binder can be poly(vinylidene fluoride) (PVDF), and can be used in both the cathode and the anode. Other binders are possible, such as carboxymethyl cellulose (CMC).

The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ can be a solid solution where the lithium concentration is in excess between 15 to 30 at %. This will form a lithium excess composition such as Li_(1+y)Ni_((1-y)(0.5-x/2))Mn_((1-y)(0.5-x/2))M_((1-y)x)O₂ where y is between 0.15-0.30 and x is between 0.005-0.02.

The usable capacity of the cathode attains 180 mAh/g when cycled between 2.0-4.5 V vs. Li/Li⁺.

A cathode composition for a lithium battery can be provided. The cathode composition includes LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of M=Mo, Ti, Cr, Zr and V, and x is 0.005-0.02. The LiNi_(0.5-x/2) Mn_(0.5-x/2) M_(x)O₂ can be coated with Mn₂P₂O₇. The Mn₂P₂O₇ can be 1-3 wt. %, based on the total weight of the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ and Mn₂P₂O₇. The LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ can be provided as particles having a diameter from 50 to 500 nm. The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ particles can be coated with Mn₂P₂O₇. The Mn₂P₂O₇ coating on the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ can cover all sides of the particles and can be from 1 to 5 nm thick.

A lithium battery according to the invention can include a cathode comprising LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is 0.005 to 0.02. The battery includes an anode, a separator, and an electrolyte.

Any suitable anode material can be used. The anode can be a mixture of graphite and a binder. The anode can material can also be Li₅Ti₄O₁₂ (LTO). Carbon additives such as include Super P Li and C65 can also be included in the anode.

Any suitable separator can be used. Porous polyolefins are common separator materials. The separator can be Celgard 2325 (Celgard LLC, Charlotte N.C.) and one sheet of glass microfiber (Whatman). Celgard 2400 and 2500 are also compatible separators.

Any suitable electrolyte can be used. The electrolyte can be 1.2 M LiPF₆ in EC-DMC in 3:7 weight ratio. LiPF₆ dissolved in liquid carbonates is a standard electrolyte system for LIBs. Two other common formulations include 1 M LiPF₆ in EC/DMC (1/1 wt ratio), and 1.2 M LiPF₆ in EC/EMC (3/7 wt ratio).

The battery can be cobalt free. The usable capacity of the cathode attains 180 mAh/g when cycled between 2.0-4.5 V vs. Li/Li⁺.

A method of making a lithium battery can include the step of providing LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is 0.005 to 0.02. The LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ is mixed with conductive carbon, a binder and a solvent to form a slurry. The slurry can be cast onto a current collector. Any suitable solvent can be used. An example of a suitable solvent is N-Methyl-2-Pyrrolidone. The slurry can be dried to form a cathode. The cathode can be combined with an anode, a separator and an electrolyte to form a battery.

FIG. 1 is a schematic drawing of the crystal structure of LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂. The M is substituted at cation sites. There is shown the LiO₆ octahedra, the MO₆ octahedra where 49-49.75% of these sites contain Mn, 49-49.75% of these sites contain Ni, and 0.5-2.0% of these sites contain an early transition metal such as Mo, Ti, Cr, Zr, and/or V.

Example I

LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂ powders were synthesized using a sol-gel procedure where Li(CH₃COO).2H₂O, Ni(OCOCH₃)₂.4H₂O, Mn(CH₃COO)₂.4H₂O, (NH4)₆Mo₇O₂₄.4H₂O, and citric acid were dissolved in deionized water in the appropriate molar ratio to obtain the desired stoichiometry. The solution was gently heated to 60° C. while stirring to slowly evaporate the water and produce a solid precursor which was heated at 400° C. (5° C./min ramp rate) for 4 h in air followed by a final heat treatment at 850° C. (5° C./min) for 15 h in air.

Electrode slurries were prepared by mixing LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂, Super P Li carbon black, and poly(vinylidene fluoride) (PVDF) (80/10/10 weight ratio) in N-Methyl-2-pyrrolidone (NMP). The slurry was cast onto a carbon-coated Al foil current collector and dried overnight before preparing electrochemical cells. CR2032 half cells were constructed in an Ar-filled glovebox using a slurry cast cathode and a Li metal counter/reference electrode. The electrolyte was 1.2 M LiPF₆ in a mixture of ethylene carbonate and ethyl methyl carbonate (3/7 by weight). The separator consisted of one sheet of Celgard 2325 and one sheet of glass microfiber (Whatman). Galvanostatic charge/discharge experiments were conducted on a MACCOR Series 4000 battery tester. Cyclic voltammograms were collected using a Biologic VSP potentiostat.

FIG. 2 is a graph of intensity (a.u) v. 2θ_(CuKα)/deg. showing the Mo doped LiNi_(0.5)Mn_(0.5)O₂, X-ray diffraction pattern. The Bragg diffraction peaks are indexed to the typical hexagonal layered α-NaFeO₂ structure (space group: R-3m). The broad peaks in the 2θ range of 20-25° are characteristic of the Li₂MnO₃ structure which belongs to a space group C2/m. Mo(1%) doped LiNi_(0.5)Mn_(0.5)O₂ was synthesized by a sol-gel process.

FIG. 3 is a SEM image of a baseline LiNi_(0.5)Mn_(0.5)O₂ without Mo doping. FIG. 4 is a SEM image of Mo doped LiNi_(0.5)Mn_(0.5)O₂. There are no major differences in the morphology of the LiNi_(0.5)Mn_(0.5)O₂ vs. the Mo-doped compound.

FIG. 5 is a graph of specific capacity (mAh/g) v. cycle number(n) for LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂, x=0 and x=0.01 and cycled between 2.0-4.5 V and at specific current density of 20 mA/g. The electrochemical properties of the materials were evaluated by a CR2032 coin-type cell with lithium foil as a reference electrode. The working electrode was prepared by a coating slurry, (80 wt %) as active materials, Super P (10 wt %) and PVDF (10 wt %) as binder within NMP, onto carbon coated aluminum foils and dried overnight at 120° C. A porous polypropylene film and glass fiber was used as separator. The Mo doped cathode shows a higher specific capacity at 20 mA/g between 2.0-4.5 V compared to the baseline.

FIG. 6 is a graph of specific capacity (mAh/g) v. cycle number (n) for LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂ x=0 and x=0.01 and at specific current density of 10-200 mA/g, and showing rate capability cycled between 2.0-4.5 V. The Mo-doped material has a higher capacity at specific current density of 10-200 mA/g.

FIG. 7 is a graph of voltage vs. Li/Li⁺/V vs. specific capacity (mAh/g) showing electrochemical characterization for baseline LiNi_(0.5)Mn_(0.5)O₂. The charge/discharge cycle at a specific current density of 20 mA/g between 2.0-4.5V is plotted. FIG. 8 is a graph of voltage vs. Li/Li₊/V vs. specific capacity (mAh/g) showing electrochemical characterization for Li Ni_(0.495)Mn_(0.495)Mo_(0.01)O₂. The charge/discharge cycle at a specific current density of 20 mA/g between 2.0-4.5 V is plotted. It can be seen comparing FIG. 7 and FIG. 8 that the presence of Mo does not impact the voltage profile.

FIG. 9 is a graph of specific capacity (mAh/g) v. cycle number (n) for LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ at 2.0-4.2 V and specific current density of 20 mA/g. The Mo doped cathode shows a stable cyclability at 20 mA/g between 2.0-4.2 V. FIG. 10 is graph of specific current (mA/g_(active material)) v. voltage (V) for LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ and x=0 and x=0.01. Cyclic voltammograms collected at 0.1 mV/s between 2.0-4.5 V are shown in FIG. 10. The cyclic voltammetry shows that the Mo-doped material shows higher peak currents compared to the unmodified material. This result is consistent with the higher capacities measured during galvanostatic charge/discharge.

FIG. 11 is a graph of normalized counts/A.U v. Raman shift/cm⁻¹ before and after 100 cycles for a baseline LiNi_(0.5)Mn_(0.5)O₂. FIG. 12 is a graph of normalized counts/A.U v. Raman shift/cm⁻¹ before and after 100 cycles for LiNi_(0.495)Mn_(0.495)Mo_(0.01)O₂. No major change in the Raman bands indicates that the structure is stable upon repeated Li insertion/extraction.

Example II

LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂ powders were synthesized using a sol-gel procedure wherein Li(CH3COO).2H₂O, Ni(OCOCH₃)₂.4H₂O, Mn(CH3COO)₂.4H₂O, (NH4)₆Mo₇O₂₄.4H₂O, and citric acid were dissolved in deionized water in the appropriate molar ratio to obtain the desired stoichiometry. The solution was gently heated while stirring to slowly evaporate the water and produce a solid precursor which was heated at 400° C. (5° C./min ramp rate) for 4 h in air followed by a final heat treatment at 850° C. (5° C./min) for 15 h in air.

To apply the Mn₂P₂O₇ coatings, LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂ powder was added under continuous stirring to a solution containing Mn(CH₃COO)₂.4H₂O dissolved in 10 mL deionized water (Suspension A). Meanwhile, a stoichiometric amount of NH₄H₂PO₄ was dissolved in 10 mL deionized water (Solution B). Solution B was added dropwise to Suspension A under continuous stirring over the course of 1 hr. The resulting suspension was dried at 60° C., and the resulting solid powder was heated at 300° C. (2° C./min ramp rate) for 5 h in air to obtain the Mn₂P₂O₇-coated LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂.

Electrode slurries were prepared by mixing LiNi_(0.5-x/2)Mn_(0.5-x/2)Mo_(x)O₂, Super P Li carbon black, and poly(vinylidene fluoride) (PVDF) (80/10/10 weight ratio) in N-Methyl-2-pyrrolidone (NMP). The slurry was cast onto a carbon-coated Al foil current collector and dried overnight before preparing electrochemical cells. CR2032 half cells were constructed in an Ar-filled glovebox using a slurry cast cathode and a Li metal counter/reference electrode. The electrolyte was 1.2 M LiPF₆ in a mixture of ethylene carbonate and ethyl methyl carbonate (3/7 by weight). The separator consisted of one sheet of Celgard 2325 and one sheet of glass microfiber (Whatman). Galvanostatic charge/discharge experiments were conducted on a MACCOR Series 4000 battery tester. Cyclic voltammograms were collected using a Biologic VSP potentiostat.

FIG. 13 is a graph of intensity (a.u) v. 2θ_(CuKα)/deg. showing synthesis of a Mn₂P₂O₇ coating on Mo doped LiNi_(0.5)Mn_(0.5)O₂. Manganese acetate was dissolved in distilled water before adding as synthesized Mo (1%) doped LiNi_(0.5)Mn_(0.5)O₂ under continuous stirring (Solution A). Meanwhile diammonium hydrogen phosphate was dissolved in distilled water (Solution B). Solution B was added to Solution A dropwise within 3 hr of continuous stirring and dried at 60° C. The dried powder obtained was heated to 300° C. for 5 hr in air atmosphere. (heating rate 2° C./min). Mn₂P₂O₇ coating is about 2 wt. %. The Bragg diffraction peaks are indexed to the typical hexagonal layered α-NaFeO₂ structure (space group: R-3m). The broad peaks in the 20 range of 20-25° are characteristic of the Li₂MnO₃ structure which belongs to a space group C2/m.

FIG. 14 is a graph of specific capacity (mAh/g) v. cycle number (n) for LNMO, LNMMO, and Mn₂P₂O₇-LNMMO for Mn₂P₂O₇ coated on an Mo doped LiNi_(0.5)Mn_(0.5)O₂ cathode, cycled between 2.0-4.5 V at a specific current density of 20-50 mA/g. The electrochemical properties of the materials were evaluated by a CR2032 coin-type cell with lithium foil as a reference electrode. The working electrode was prepared by coating slurry, (80 wt %) as active materials, Super P (10 wt %) and PVDF (10 wt %) as binder within NMP, onto carbon coated aluminum foils and dried overnight at 120° C. A porous polypropylene film and glass fiber was used as separator. The electrolyte(Gen 2) was composed of 1.2 M LiPF₆ dissolved in a mixed organic solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC) (3:7 in weight ratio). The coin-type cells were assembled in an Ar-filled glovebox. The Mn₂P₂O₇-coated cathode shows higher specific capacity when cycled between 2.0-4.5 V vs. Li/Li⁺ compared to the Mo doped LNMMO and undoped LNMO cathodes.

FIG. 15 is a graph of specific capacity (mAh/g) v. cycle number (n) for LNMO, LNMMO, and Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂ cathode, cycled between 2.0-4.2 V vs. Li/Li⁺ and a specific current density of 20 mA/g. The Mn₂P₂O₇ coated cathode shows similar performance to the Mo doped cathode at 20 mA/g between 2.0-4.2 V.

FIG. 16 is a graph of specific capacity (mAh/g) v. cycle number (n) for Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂, cycled at 2.0-4.5 V vs. Li/Li⁺ and a specific current of 20 mA/g. The Mn₂P₂O₇ coated cathode shows higher specific capacity retention when cycled between 2.0-4.5 V vs. Li/Li⁺ compared to the Mo doped cathode and baseline.

FIG. 17 is a graph of specific capacity (mAh/g) v. cycle number for LNMO, LNMMO, and Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂, cycled between 2.0-4.5 V vs. Li/Li⁺ and a specific current density of 10-200 mA/g. The Mn₂P₂O₇ coated cathode shows higher specific capacity retention when cycled between 2.0-4.5 V at specific current density of 10-200 mA/g. It shows higher performance due to the Mn₂P₂O₇ coating. The coating layer mitigates detrimental side reactions at the electrode/electrolyte interface. The coating layer also preserves the smooth surface of the active material upon cycling and maintain the structural integrity. The coating layer also increased structural stability due to the strong covalent phosphate bonding. The coating layer may enhance the apparent lithium ion diffusion coefficient and as a result improve de-lithiation kinetics.

FIG. 18 is a graph of potential (V vs. Li/Li⁺) v. specific capacity mAh/g Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂. The charge/discharge 1^(st) cycle at 20 mA/g between 2.0-4.5 V vs. Li/Li⁺ is shown for the Mn₂P₂O₇ coated, baseline and Mo doped cathode materials. These are LiNi_(0.5)Mn_(0.5)O₂-LNMO, LiNi_(0.495)Mn_(0.495)Mo_(0.01)O₂-LNMMO, and Mn₂P₂O₇ coated LiNi_(0.495)Mn_(0.495)Mo_(0.01)O₂—Mn₂P₂O₇-LNMMO. The Mn₂P₂O₇ coating impacts the voltage profile. It shows the redox peak at about 3.0 V vs. Li/Li⁺ due to the Mn³⁺/Mn⁴⁺ redox couple.

FIG. 19 is a graph of specific capacity (mAh/g) v. cycle number (n) Mn₂P₂O₇ coated on Mo doped LiNi_(0.5)Mn_(0.5)O₂ cycled between 2.0-4.2 V, 2.0-4.5 V, 2.0-4.6 V, and 2.0-4.8 V. At higher voltage cut-offs, the electrolyte is decomposed. However, at 4.5 V the electrode shows stable electrochemical performance.

The invention is not limited in its application to the details of construction and to the arrangement of the components set forth in the foregoing description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. In addition, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

It is to be understood that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. 

We claim:
 1. A cathode for a lithium battery, comprising LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is between 0.005-0.02.
 2. The cathode of claim 1, wherein the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ is coated with Mn₂P₂O₇.
 3. The cathode of claim 2, wherein the Mn₂P₂O₇ is 1-3 wt. %, based on the total weight of the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ and Mn₂P₂O₇.
 4. The cathode of claim 1, wherein the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ is provided as particles having a diameter of from 50 to 500 nm.
 5. The cathode of claim 4, wherein the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ is coated with Mn₂P₂O₇, and the coating is between 1 and 5 nm thick.
 6. The cathode of claim 1, wherein M is Mo.
 7. The cathode of claim 1, wherein the cathode is cobalt free.
 8. The cathode of claim 1, wherein M is substituted at cation sites.
 9. The cathode of claim 1, further comprising a conductive carbon and a binder.
 10. The cathode of claim 1, wherein the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ is a solid solution wherein the lithium concentration is in excess between 15 to 30 at % to form a lithium excess composition.
 11. The cathode of claim 1, wherein with usable capacity of the cathode attains 180 mAh/g when cycled to 4.5 V vs. Li/Li⁺.
 12. A cathode composition for a lithium battery, comprising LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of M=Mo, Ti, Cr, Zr and V, and x is 0.005-0.02.
 13. The cathode composition of claim 12, wherein the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, is provided as particles, and the particles are coated with Mn₂P₂O₇.
 14. The cathode composition of claim 13, wherein the Mn₂P₂O₇ coating is 1-3 wt %, based on the total weight of the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ and Mn₂P₂O₇.
 15. The cathode composition of claim 13, wherein the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ is provided as particles having a diameter of from 50 to 500 nm.
 16. The cathode composition of claim 15, wherein the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ is coated with Mn₂P₂O₇, and the coating is between 1 and 5 nm thick.
 17. The cathode of claim 1, further comprising conductive carbon and a binder.
 18. A lithium battery, comprising: a cathode comprising LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is 0.005 to 0.02; an anode; a separator; and an electrolyte.
 19. The battery of claim 18, wherein the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, is provided as particles, and the particles are coated with Mn₂P₂O₇.
 20. The battery of claim 19, wherein the Mn₂P₂O₇ coating is 1-3 wt %, based on the total weight of the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ and Mn₂P₂O₇.
 21. The battery of claim 18, wherein the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ is provided as particles having a diameter of from 50 to 500 nm.
 22. The battery of claim 21, wherein the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ is coated with Mn₂P₂O₇, and the coating is between 1 and 5 nm thick.
 23. The battery of claim 18, wherein the cathode is cobalt free.
 24. The battery of claim 18, wherein M is substituted at cation sites.
 25. The battery of claim 18, wherein with usable capacity of the cathode attains 180 mAh/g when cycled between 2.0-4.5 V vs. Li/Li⁺.
 26. A method of making a lithium battery, comprising the steps of: providing LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂, where M is at least one selected from the group consisting of Mo, Ti, Cr, Zr and V, and x is 0.005 to 0.02; mixing the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ with conductive carbon, a binder and a solvent to form a slurry, and casting the slurry onto a current collector and drying the slurry to form a cathode; combining the cathode with an anode, a separator and an electrolyte to form a battery.
 27. The method of claim 26, wherein the LiNi_(0.5-x/2) Mn_(0.5-x/2)M_(x)O₂ is provided as particles having a diameter of from 50 to 500 nm.
 28. The method of claim 27, wherein the LiNi_(0.5-x/2)Mn_(0.5-x/2)M_(x)O₂ particles are coated with Mn₂P₂O₇.
 29. The method of claim 28, wherein the coating is between 1 and 5 nm thick.
 30. The method of claim 26, wherein the binder is PVDF.
 31. The method of claim 26, wherein the solvent is N-Methyl-2-Pyrrolidone (NMP).
 32. The method in claim 26, wherein the electrolyte is 1.2 M LiPF₆ in EC-EMC in 3:7 weight ratio.
 33. The method in claim 26, wherein the anode composition is a mixture of graphite and a binder. 